No, 4130 isn't what ALL race car roll cages and subframes are made out of. MOST of them are made out of DOM which stands for Drawn Over Mandrel for those of you who don't know. 4130 is used by racers that can AFFORD it.

DOM is a manufacturing process, not a steel grade.

4130 is DOM. The least expenisve DOM grade is a plain carbon steel such as ASTM 1018 or 1020, which would also work exceptionally well in this application. I guess I am just used to 4130 because you can realize a weight savings over plain carbon by decreasing the wall thickness due to the higher strength of the material.

Either way, steel tubing would be your best choice, preferrably a DOM or seamless tube.